The present invention relates to an adaptive antenna reception apparatus for receiving code division multiple access (CDMA) signals and adaptively forming antenna directivity beams to receive a desired user signal while suppressing other user interference signals.
The CDMA scheme has the potential of increasing the subscriber capacity and is expected as a radio access scheme for a next-generation mobile communication cellular system. On the base station receiving side, however, a problem arises in terms of interference with other user signals due to simultaneous access with the same carrier. As a method of eliminating such interference, a method using an array antenna is available.
An array antenna receives signals through a plurality of antennas and performs weighted combining of complex numbers. With this operation, the amplitude and phase of the signal received through each antenna are controlled to form a directivity beam so as to receive a desired user signal, and other user interference signals are suppressed. An adaptive array antenna is an apparatus for forming such a directivity beam by adaptive control.
FIG. 6 shows a conventional adaptive antenna reception apparatus. Referring to FIG. 6, the adaptive antenna reception apparatus has L path processing sections 107-1 to 107-L corresponding to the number of paths in accordance with multipath propagation channels in a mobile communication environment. The adaptive antenna reception apparatus forms directivity beams for the respective L paths to receive signals therefrom, and combines the reception signals to obtain a high-quality reception signal. The path processing sections 107-1 to 107-L have the same arrangement. Each section is comprised of a beam former 101 and adaptive control section 106.
The beam former 101 receives a despread signal obtained at each path timing by using the spreading code of a desired user, and performs weighted combining of complex numbers to form a directivity beam for each path.
A combiner 102 combines outputs from the respective beam formers 101. A determination unit 103 determines a transmission signal with the highest possibility of being a reception signal, and outputs a user determination symbol. A switch 104 is switched to output a known reference signal as a reference signal to a subtracter 105 if the known reference signal is present and to output a determination signal as a reference signal to the subtracter 105 if no known reference signal is present. The subtracter 105 generates an error signal by subtracting the reception signal sent from the combiner 102 from the reception signal sent from the switch 104.
The adaptive control section 106 obtains an antenna weighting factor used by each beam former 101 by adaptive control using an error signal and a reception signal before beam forming. Adaptive control, minimum mean square error (MMSE) control is generally used. As adaptive update algorithms for antenna weighting factors using error signal, LMS (Least Mean Square), NLMS (Normalized LMS), and RLS (Recursive Least Square) algorithm are known.
FIG. 9 shows the gains of beam patterns formed by the beam formers for the respective paths. FIG. 9 shows a case of two paths. In this case, high gains are directed to the respective path directions of desired signals, while the gains in the interference signal directions are suppressed low. In an array antenna using adaptive control, a beam is directed to a desired signal direction, and a point (null) where the gain is extremely low is directed to an interference direction to maximize the reception SINR (Signal to Interference and Noise Ratio).
Other conventional adaptive antenna reception apparatuses are disclosed in xe2x80x9cPilot Symbol-Assisted Decision-Directed Coherent Adaptive Array Diversity for DS-CDMA Mobile Radio Reverse Linkxe2x80x9d, IEICE Trans. vol. E80-A, pp. 2445-2454, December 1997 (reference 1) and xe2x80x9cPerformance of Coherent Adaptive Antenna Array Diversity Receiver Using the Common Antenna Weights for Rake Combined Paths for W-CDMA Reverse Linkxe2x80x9d, Technical Report of IEICE, RCS 99-100, August 1998 (reference 2).
FIG. 7 shows the basic arrangement of this scheme. A beam former 121 receives a despread signal obtained at each path timing by using the spreading code of a desired user, and performs weighted combining of complex numbers to form a directivity beam for each path. A combiner 122 combines outputs from the respective beam formers 121. A determination unit 123 determines a transmission signal with the highest possibility of being a reception signal.
A switch 124 is switched to output a known reference signal as a reference signal to a multiplier 125 if the known reference signal is present and to output a determination signal as a reference signal to the multiplier 125 if no known reference signal is present. The multiplier 125 multiplies the reference signal output from the switch 124 and the reference signal level output from an averaging section 130. A subtracter 126 generates an error signal by subtracting the reception signal from the output from the multiplier 125.
An adaptive control section 127 obtains an antenna weighting factor used by the beam former 121 by adaptive control using the error signal output from the multiplier 125 and the reception signal before beam forming. According to the above reference, antenna weighting factors are updated by using the NLMS algorithm.
A reference signal level A(n) is calculated on the basis of the transmission channel estimation value output from the beam former 121. A reception level detection section 128 detects the reception level of each path from a corresponding transmission channel estimation value, and outputs it to an adder 129. The adder 129 adds the reception levels of the respective paths output from the reception level detection section 128. The averaging section 130 averages the sum level output from the adder 129 and outputs the resultant value to the multiplier 125. The presence/absence of the averaging section 130 or its averaging time is arbitrarily determined.
FIG. 8 shows an example of the reception level detection section 128. Referring to FIG. 8, an amplitude detection section 141 detects the absolute value (amplitude) of a transmission channel estimation value (complex number). A squaring section 142 calculates the square (power) of an output from the amplitude detection section 141. The reference signal level A(n) is given by                               A          ⁡                      (            n            )                          =                              ∑                          n              =              0                                      N              AVR                                ⁢                      xe2x80x83                    ⁢                                    ∑                              i                =                0                                            L                -                1                                      ⁢                          xe2x80x83                        ⁢                                          "LeftBracketingBar"                                  h                  ⁡                                      (                                          i                      ,                      n                                        )                                                  "RightBracketingBar"                            2                                                          (        1        )            
where NAVR is the averaging time of the averaging section 130.
The conventional adaptive antenna reception apparatus exhibits excellent steady-state characteristics owing to low-speed MMSE adaptive control in which antenna weighting factors are long-term-averaged. In a high-speed fading environment or the like, amplitude and phase variations due to fading cannot be corrected. For this reason, to correct phase variations, a transmission channel correction section is connected to the output terminal of each beam former.
With regard to reception level variations, however, when a prescribed value is used as a reference signal, a level difference is produced between the reception signal and the reference signal, resulting in an error in the desired signal itself. This leads to unstable operation. If high-speed transmitting power control (TPC) is used together, a desired signal is suppressed in a short period of time in the process of initial convergence, and TPC diverges.
According to a method of determining a reference signal in accordance with a reception level, when a reception level after beam forming is used as a reference signal, the reference signal level drops as the beam gain decreases, as in the prior art. For this reason, the gain decreases for a long term because of the absence of power that adjusts the gain direction of a beam. In this method, therefore, the capability of tracking a change in the arriving direction of a desired signal may deteriorate.
This tracking capability can be improved by averaging reference signal levels. This operation amounts to setting the averaging time NAVR of signal levels to be long. If this averaging time is set to be long, variations in reception level cannot be quickly handled. This method is therefore equivalent to the method using a prescribed value.
It is an object of the present invention to provide an adaptive antenna reception apparatus which can stabilize adaptive control by quickly changing a reference signal in accordance with a reception level.
In order to achieve the above object, according to the present invention, there is provided an adaptive antenna reception apparatus for receiving multiplex signals from a plurality of users with the same carrier through a plurality of antennas, and receiving a desired user signal by adaptively forming an antenna directivity beam while suppressing other user interference signals, comprising determination means for determining a level of a reference signal for generating an error signal used for antenna adaptive control on the basis of a reception level of each antenna before beam forming, a plurality of adaptive control means for calculating antenna weighting factors for the respective paths on the basis of the multiplex signals received through the respective antennas and the error signal output from the determination means, and a plurality of beam formers for receiving the received multiplex signals and forming antenna directivity beams for the respective paths in accordance with the antenna weighting factors output from the adaptive control means.